Automatic bias control method and apparatus for optical transmitter

Abstract

The embodiments provide an automatic bias control method and apparatus for an optical transmitter. The apparatus includes: a detecting unit configured to monitor output optical power of an I/Q modulator of the optical transmitter; a calculating unit configured to calculate bias voltage indicating values of the I modulator, Q modulator and phase modulator of the I/Q modulator according to the output optical power and known modulation data; and an adjusting unit configured to adjust respectively Direct-Current (DC) bias voltages of the I modulator, Q modulator and phase modulator according to the bias voltage indicating values of the I modulator, Q modulator and phase modulator. With the embodiments, known modulation data are used to realize automatic bias control by monitoring the evenness of distribution of the power of output optical signals of the transmitter in the four quadrants of an I/Q plane.

What is claimed is: 
     
       1. An automatic bias control apparatus for optical transmitter, comprising:
 a detecting unit configured to monitor output optical power of an I/Q modulator of the optical transmitter; 
 a calculating unit configured to calculate respective bias voltage indicating values of the I modulator, Q modulator and phase modulator of the I/Q modulator according to the output optical power and known modulation data; and 
 an adjusting unit configured to adjust respectively Direct-Current (DC) bias voltages of the I modulator, Q modulator and phase modulator according to the respective bias voltage indicating values of the I modulator, Q modulator and phase modulator; 
 wherein the calculating unit comprises: 
 a sign extracting module configured to extract the sign of I modulation data of the I/Q modulator, the sign of Q modulation data of the I/Q modulator, and the sign of a product of the I modulation data and the Q modulation data; and 
 a calculating module configured to calculate respectively the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the output optical power, the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data. 
 
     
     
       2. The apparatus according to  claim 1 , wherein
 the calculating module calculates respectively the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the formulae below:
   Δ I =mean( P ·sign( D   I ));
 
   Δ Q =mean( P ·sign( D   Q ));
 
   Δ Φ =−mean( P ·sign( D   I   *D   Q ));
 
 
 and the adjusting unit adjusts respectively the DC bias voltage of the I modulator, the DC bias voltage of the Q modulator and the DC bias voltage of the phase modulator according to the formulae below:
     V   I   =V   I −μ I Δ I ;
 
     V   Q   =V   Q −μ Q Δ Q ;
 
     V   φ   =V   φ −μ φ Δ φ ;
 
 
 where, P is the output optical power, D I  is the I modulation data of the I/Q modulator, D Q  is the Q modulation data of the I/Q modulator, D I *D Q  is the product of the I modulation data and the Q modulation data, sign(•) denotes sign extracting operation, sign(D I ) is the sign of the I modulation data, sign(D Q ) is the sign of the Q modulation data, sign(D I *D Q ) is the sign of the product of the I modulation data and the Q modulation data, mean(•) denotes averaging operation, Δ I  is the bias voltage indicating value of the I modulator, Δ Q  is the bias voltage indicating value of the Q modulator, Δ Φ  is the bias voltage indicating value of the phase modulator, and μ I , μ Q , μ φ  are respective step sizes for adjusting V I , V Q , V Φ , which are positive real numbers. 
 
     
     
       3. The apparatus according to  claim 1 , wherein the calculating unit further comprises:
 a first low-pass filter module configured to perform low-pass filtering to the output optical power; and/or 
 a second low-pass filter module configured to perform low-pass filtering to the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data; 
 and/or 
 a first analog-to-digital converting module configured to perform analog-to-digital conversion to the output optical power; and 
 a second analog-to-digital converting module configured to perform analog-to-digital conversion to the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data; 
 and/or 
 a tunable delay module configured to tune respectively time delay of the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data. 
 
     
     
       4. The apparatus according to  claim 3 , wherein
 when the calculating unit comprises the first low-pass filter module, the calculating module calculates the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the low-pass filtered output optical power, the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data; 
 when the calculating unit comprises the second low-pass filter module, the calculating module calculates the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the output optical power, the low-pass filtered sign of the I modulation data, the low-pass filtered sign of the Q modulation data and the low-pass filtered sign of the product of the I modulation data and the Q modulation data; and 
 when the calculating unit comprises the first low-pass filter module and the second low-pass filter module, the calculating module calculates the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the low-pass filtered output optical power, the low-pass filtered sign of the I modulation data, the low-pass filtered sign of the Q modulation data and the low-pass filtered sign of the product of the I modulation data and the Q modulation data. 
 
     
     
       5. The apparatus according to  claim 1 , wherein the calculating unit further comprises:
 a first analog-to-digital converting module configured to perform analog-to-digital conversion to the output optical power; and 
 a down sampling module configured to perform respectively down sampling to the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data. 
 
     
     
       6. The apparatus according to  claim 5 , wherein the calculating unit further comprises:
 a first low-pass filter module configured to perform low-pass filtering to the output optical power; and/or 
 a tunable delay module configured to tune respectively time delay of the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data. 
 
     
     
       7. An optical transmitter, comprising an I/Q modulator and an automatic bias control apparatus as claimed in  claim 1 . 
     
     
       8. An automatic bias control method for optical transmitter, comprising:
 monitoring output optical power of the I/Q modulator of the optical transmitter; 
 calculating respectively bias voltage indicating values of I modulator, Q modulator and phase modulator of the I/Q modulator according to the output optical power and known modulation data; and 
 adjusting respectively DC bias voltages of the I modulator, Q modulator and phase modulator according to the respective bias voltage indicating values of the I modulator, Q modulator and phase modulator; 
 wherein the step of calculating bias voltage indicating values comprises: 
 extracting the sign of I modulation data of the I/Q modulator, the sign of Q modulation data of the I/Q modulator, and the sign of product of the I modulation data and the Q modulation data; and 
 calculating respectively the bias voltage indicating values of the I modulator, Q modulator and phase modulator according to the output optical power, the sign of the I modulation data, the sign of the Q modulation data and the sign of the product of the I modulation data and the Q modulation data.